Prevent type declaration files to generate pages (#3076)

packages/gatsby/src/internal-plugins/component-page-creator/__tests__/gatsby-node.js

@@ -34,6 +34,25 @@ describe(`JavaScript page creator`, () => {
     expect(files.filter(file => validatePath(file.path)).length).toEqual(1)
   })
 
+  it(`filters out files that have TypeScript declaration extensions`, () => {
+    const files = [
+      {
+        path: `something/foo.ts`,
+      },
+      {
+        path: `something/bar.tsx`,
+      },
+      {
+        path: `something/declaration-file.d.ts`,
+      },
+      {
+        path: `something-else/other-declaration-file.d.tsx`,
+      },
+    ]
+
+    expect(files.filter(file => validatePath(file.path)).length).toEqual(2)
+  })
+
   describe(`create-path`, () => {
     it(`should create unix paths`, () => {
       const basePath = `/a/`

packages/gatsby/src/internal-plugins/component-page-creator/validate-path.js

@@ -1,11 +1,15 @@
 const systemPath = require(`path`)
 
+const tsDeclarationExtTest = /\.d\.tsx?$/
+
 module.exports = path => {
   // Disallow paths starting with an underscore
   // and template-.
+  // and .d.ts
   const parsedPath = systemPath.parse(path)
   return (
     parsedPath.name.slice(0, 1) !== `_` &&
-    parsedPath.name.slice(0, 9) !== `template-`
+    parsedPath.name.slice(0, 9) !== `template-` &&
+    !tsDeclarationExtTest.test(parsedPath.base)
   )
 }